Heating filament assembly and a method of preparing same



y 26, 1954 J. KoPEcKi 3,134,691

HEATING FILAMENT ASSEMBLY AND A METHOD OF PREPARING SAME Filed Oct. 3, 1961 INVENTOR. Iii); Iibpec/(j United States Patent 3,134,691 HEATING FILAMENT ASSEMBLY AND A METHOD OF PREPARING SAD m Jiii Kopecky, Prague, Czechoslovakia, assignor to Tesla, na'rodni podnik, Prague, Czechoslovakia Filed Oct. 3, 1961, Ser. No. 142,508 Claims priority, application Czechoslovakia Oct. 18, 1960 4 Claims. (Cl. 117215) This invention relates to a heating filament assembly, and more particularly to such an assembly for electron tubes, and it also relates to a method of preparing the heating filament assembly.

It is generally known to use in the production of electron tubes aluminium oxide coatings for the insulation of heating filaments. These coatings are completely satisfactory as far as oxide cathodes are concerned because these cathodes are operated at a temperature within the range from 800 to 850 C. If it is assumed that the temperature gradient between the cathode and the heating filament is within the range from 300 to 400 C., it can be concluded that the temperature of the heating filament reaches at the most 1300" C. The thermal resistance of aluminum oxide is at these conditions fully sufficient. With the increasing demands on the output of electron tubes, new types of cathodes have been developed by whose 'em'issive power the efiiciency of oxide cathodes is increased many times. The new types of cathodes are, for example, boride cathodes and especially impregnated cathodes. However, they require for their operation much higher temperatures. With impregnated cathodes, this temperature is about 1,150 C., While the activation temperature is 100 C. to 200 C. higher. Also, the holders of the cathodes, due to the required high vibration resistivity, are to be produced of rigid materials, due to which the temperature conduction of the cathode is increased, so that the temperature difference between the cathode and the filament is increased to a value of 500- 600 C. The temperature of the heating filament thus increases to a value of 1,700 to 1,850 C.

The melting point of aluminium oxide is at 2,050 C. It has been found that on contact of aluminium oxide with metallic tungsten, during a long lasting heating process in vacuum at a temperature of approx. 1,700" C., aluminium oxide is slowly reduced to metallic aluminium, while tungsten changes to tungsten trioxide which reacts together with aluminium oxide to form other compounds, hesion of the aluminium oxide layer sintered on the tungsten filament decreases, and the melting point of aluminium oxide is at the same time successively lowered so that the insulating layer will melt and evaporate at the above indicated temperature within 12 to 400 hours.

The aforesaid difiiculties are eliminated by the present invention according to which the heating filament is covered with two layers or coatings. The first coating is on the filament and consists essentially of aluminium oxide, beryllium oxide, or mixtures thereof. The second coating is on the first coating and consists essentially of a major amount, but less than 96% by volume, of a first component and a minor amount of a second component. The first component is aluminum oxide, beryllium oxide, or mixtures thereof, While the second component is titanium oxide, zirconium oxide, hafnium oxide, thorium oxide, chromium oxide, molybdenum oxide, tungsten oxide, uranium oxide, or mixtures thereof.

Insulating material W Mo Ta Aluminium oxide. 1, 700-1,750 1, 700-1, 750 18, 000-1, 850 Beryllium 0xide 1, 750-1, 800 1, 700 1, 600 Magnesium oxide 1 1,750 1, 550-1, 600 1, 700-1,800 Zirconium dioxide... 1, 4001, 500 2 1, 700-1, 750 1, 000 Thorium dioxide. i 2, 000 1, 700 1, 8504, 900

1 Beginning of evaporation. 2 Mo fumes.

From the above listed values it is apparent that the use of tungsten and aluminium oxides and/or beryllium oxide is most advantageous. The upper limit of 1,700 to 1,750 C. for the temperature of the heating filament remains, however, unchanged. It is therefore necessary to redesign the cathode and the heating filament so that this temperature of the filament may not be exceeded.

According to my invention, the temperature difference between the cathode and the filament is decreased by 200 to 250 C., due to which the temperature of the filament is lowered to 1,5001,650 C., i.e., below the limit where there arises the danger of reaction between tungsten and aluminium or beryllium oxide. The increased radiation coefiicients of some substances is utilized to essentially increase the volume of radiated heat and to considerably cool down the heating filament.

The present invention will now be described in greater detail.

The heating filament is covered with a sintered insulating substance of aluminium or beryllium oxide or their mixture. This insulating substance is covered by spraying, immersion, coating or by electrophoresis with a layer consisting of a mixture of two components. The latter layer is then sintered at a temperature which is lower than 1,650 C. either in vacuum or in an electric furnace in a stream of dry hydrogen or carbon monoxide. One of the referred to two components is aluminium oxide or beryllium oxide or their mixture, whereby the volume of this component is less than 96% of the mixture. The other of the two components of the mixture is either a single oxide or a mixture of two or several oxides of the following metals: titanium, zirconium, hafnium, thorium, chromium, molybdenum, tungsten, and uranium.

The second layer or coating is applied as follows. For example a suspension of aluminium oxide and titanium oxides in methylated spirit, capable of being coated by electrophoresis, is first prepared. Heating filaments covered with a first coating of sintered aluminum oxide are now covered with a second coating of aluminum oxide and titanium oxide to a thickness of approximately 30 The thus coated filaments are sintered in an electric furnace at a temperature of 1,350 C. in a stream of dry hydrogen through 5 minutes. During this sintering, reduction of titanium dioxide to titanum sesquioxide which is black-blue in color occurs. The thermal radiation coefificient is approximately 0.80 as compared with 0.30 with the formerly used aluminium oxide. In order to attain a higher efficiency, the interior wall of the cathode holder may also be coated and sintered in the same way. The temperature difference between the cathode and the heating filament is thus decreased to 350 C.

The specification is accompanied by a drawing the only figure of which shows a heating filament according to the invention, partly broken away.

Referring to the drawing in detail now, a conductor 1 of a filament 4 is covered with a layer 2 consisting essentially of electrically insulating aluminum oxide. This layer, in turn, is coated with a layer 3 of a mixture of beryllium oxide and zirconium oxide, which mixture constitutes a high efliciency thermal radiation material. The beryllium oxide is present in an amount of less than 96% by volume of the mixture.

The method of my invention can be applied to cathodes with a high working temperature as well as to oxide cathodes. In both cases, there is a decrease of the temperature difference between cathode and filament and of the temperature of the heating element proper. The invention makes it possible to preserve the structure and composition of the filament, to maintain the filament voltage, and to save tungsten. Another advantage is that due to the increase of the radiation capability of my filament even temperature differences among cathodes of individual electron tubes, which normally occur due to nonuniform coiling of the filaments of the cathodes, are eliminated to a minimum.

It will be apparent that many changes and modifications may be made which will not depart from the scope of the invention, as defined in the appended claims.

What I claim is:

1. A heating filament assembly for electron tubes, comprising a conductive means; an electrically insulating first coating on said conductive means, said first coating es sentially consisting of a material selected from the group consisting of aluminum oxide, beryllium oxide, and mixtures thereof; and a second coating on said first coating, said second coating essentially consisting of a mixture of a major amount, but less than 96% by volume, of a first component and of a minor amount of a second component, said first component being selected from the group consisting of aluminum oxide, beryllium oxide, and mixtures thereof, said second component being selected from the group consisting of titanium oxide, zirconium oxide, hafnium oxide, thorium oxide, chromium oxide, molybdenum oxide, tungsten oxide, uranium oxide, and mixtures thereof.

2. In the filament assembly according to claim 1, said second component being a mixture of at least two of said second component oxides.

3. A method of preparing a heating filament assembly for electron tubes, which comprises applying a high efficiency thermal radiation coating to a conductive means provided with an electrically insulating coating, said thermal radiation coating being a mixture of a major amount, but less than 96% by volume, of a first component and of a minor amount of a second component, said first component being selected from the group consisting of aluminum oxide, beryllium oxide, and mixtures thereof, said second component being selected from the group consisting of titanium oxide, zirconium oxide, hafnium oxide, thorium oxide, chromium oxide, molybdenum oxide, tungsten oxide, uranium oxide, and mixtures thereof, said application being carried out in vacuum.

4. A method of preparing a heating filament assembly for electron tubes, which comprises applying a high efficiency thermal radiation coating to a conductive means provided with an electrically insulating coating, said thermal radiation coating being a mixture of a major amount, but less than 96% by volume, of particles of a first component selected from the group consisting of aluminum oxide, beryllium oxide, and mixtures thereof, and of particles of titanium dioxide, and sintering said particles in a stream of dry hydrogen until a portion of said titanium dioxide is reduced to titanium sesquioxide.

References Cited in the file of this patent UNITED STATES PATENTS 212,860 Du Motay Mar. 4, 1879 1,821,359 Reerink et al. Sept. 1, 1931 2,128,270 Spanner et al. Aug. 30, 1938 2,734,857 Snyder Feb. 14, 1956 2,985,548 Blickwedel et al May 23, 1961 3,029,360 Etter Apr. 10, 1962 3,041,210 Mayer June 26, 1962 FOREIGN PATENTS 684,141 Great Britain Dec. 10, 1952 

3. A METHOD OF PREPARING A HEATING FILAMENT ASSEMBLY FOR ELECTRON TUBES, WHICH COMPRISES APPLYING A HIGH EFFICIENCY THERMAL RADIATION COATING TO A CONDUCTIVE MEANS PROVIDED WITH AN ELECTRICALLY INSULATING COATING, SAID THERMAL RADIATION COATING BEING A MIXTURE OF A MAJOR AMOUNT, BUT LESS THAN 96% BY VOLUME, OF A FIRST COMPONENT AND OF A MINOR AMOUNT OF A SECOND COMPONENT, SAID FIRST COMPONENT BEING SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE, BERYLLIUM OXIDE, AND MIXTURES THEREOF, SAID SECOND COMPONENT BEING SELECTED FROM THE GROUP CONSISTING OF TITANIUM OXIDE, ZIRCONIUM OXIDE, HAFNIUM OXIDE, THORIUM OXIDE, CHROMIUM OXIDE, MOLYBDENUM OXIDE, TUNGSTEN OXIDE, URANIUM OXIDE, AND MIXTURES THEREOF, SAID APPLICATION BEING CARRIED OUT IN VACUUM. 